A gypsum board is known as a board having a gypsum core covered with sheets of paper for gypsum board liner. The gypsum boards are widely used in various kinds of buildings as architectural interior finish materials because of their advantageous fire-resisting or fire-protecting ability, sound insulation performance, workability, cost performance and so on. In general, the gypsum boards are produced by a continuous pouring and casting process. This process comprises a mixing and stirring step of admixing calcined gypsum, adhesive auxiliary agent, set accelerator, foam (or foaming agent), and so forth with a quantity of mixing water in a mixer; a forming step of pouring calcined gypsum slurry prepared in the mixer (referred to as “slurry” hereinafter) into a space between sheets of paper for gypsum board liner and forming them into a continuous plate-like belt form; and a drying and cutting step of roughly cutting the solidified continuous belt-like layered formation, drying it forcibly and thereafter, trimming it to be a product size.
Usually, a thin and circular pin-type mixer (also called as a “centrifugal pin-type mixer”) is used as the mixer for preparing the slurry in the gypsum board production process and so forth. This type of mixer comprises a flattened circular housing and a rotary disc rotatably positioned in the housing, as disclosed in, for example, PCT Pamphlet of PCT International Application No. WO 00/56435 (Patent Literature 1). A rotary driving device is located above the housing. A rotary shaft of the rotary driving device extends through a center part of the upper cover or upper plate of the housing. The shaft is fixed to a center part of the rotary disc. The upper plate of the housing is equipped with a plurality of upper pins (stationary pins). The upper pins depend from the upper plate down to the vicinity of the rotary disc. The rotary disc is equipped with lower pins (movable pins). The lower pins are vertically fixed on the disc and extend up to the vicinity of the upper plate. The upper and lower pins are arranged in radially alternate positions. A plurality of ingredient feeding ports for feeding the aforementioned materials into the mixer are disposed in a center region of the top cover or upper plate of the housing. The materials to be mixed and kneaded are supplied onto the disc through the respective feeding ports. The materials are mixed and kneaded while being moved radially outward on the disc under an action of centrifugal force. A slurry discharge port for delivering the mixture (slurry) out of the mixer is provided on a periphery of the housing or a lower plate (bottom plate) thereof. The slurry is delivered out of the mixer through the slurry discharge port.
As another type of mixer, a scraper-type mixer is known in the art. This type of mixer stirs the ingredients to be mixed with the use of a rotary disc and a scraper. For example, the mixer as disclosed in Japanese Patent Laid-Open Publication No. 7-1437 (Patent Literature 2) comprises a flattened circular housing and a rotary disc rotatably positioned in the housing, similarly to the pin-type mixer as set forth above. A rotary driving device is located below the housing. A rotary shaft of the rotary driving device extends through a center part of the lower plate (bottom plate) of the housing. The shaft is fixed to a center part of the rotary disc. A scraper is attached to a lower surface of the disc. Furthermore, another scraper is positioned under an upper cover or upper plate, in the vicinity of its underside surface. The upper and lower scrapers rotate together with the rotating disc. The materials to be mixed and kneaded and the mixing water are supplied onto the disc through respective feeding ports of the upper cover or plate. The materials and water are stirred and mixed while being moved radially outward on the disc under an action of centrifugal force, and then, are delivered out of the mixer through a slurry discharge port.